He decides it is time to give up the criminal life, but just when you thought you were out, they pull you back in again. He owes it to his buddies. He has to protect his new girlfriend from the threats of the evil Pete Postlethwaite. Once you're in the 'life,' it is next to impossible to get out. All of this goes a long way to creating questionable sympathy for the character, and it is questionable. He is after all a criminal. He never has any qualms about hurting innocent bystanders when they run from the police in some really elaborate car chases. He has no trouble firing automatic weapons into crowds of people. This is not a nice guy. There may be reasons why he is not a nice guy, but there is no question he is not a nice guy, and it seems wrong to create empathy for him.
The contrast between Affleck's conflicted MacRay, Renner's 'mad dog' portrayal of his buddy in crime James Coughlin only serves to reinforce these misplaced feelings. Coughlin is a vicious killer with a hair-trigger temper much in the tradition of Sonny Corleone or even Joe Pesci's Tommy DeVito. Like them, he acts instinctively, little thinking or caring about the consequences of his actions. Leaving aside the question of whether or not Renner's performance is nuanced enough to deserve an award (he is after all is said and done a hell of a long shot), he has created a character that makes Affleck's MacRay look like an angel in comparison. By the way, his last sip at the soda container, before he staggers out into a police barrage is classic, although it does put me in mind of James Cagney in White Heat.
In the end even if you can buy the idea of a bunch of Boston boys pulling the crime of the century against the sainted Red Sox, you have to wonder about the moral posture that seems to justify at least one of them from getting away with it. I recognize that in real life bad guys don't always get punished. But not punishing them is not quite the same thing as suggesting their rehabilitation by anonymously using their ill gotten gains to refurbish a hockey rink for neighborhood kids, leaving the "hero" lounging on a semi-tropical beach staring out at the ocean with life (and presumably love) all before him.